The 3 fileds values are the same !
It was the first thing I've checked ... I excuted an external query to count record having a count >1
select a.field1, a.field2, a.field3, count(*)
select field1, field2, field3, sum( field6 ), sum( field7 )
group by field1, field2, field
group by a.field1, a.field2, a.field3
No records found !
Then I tried to filter using a set of field1, field2, field3 values causing the PK error.
With this smaller dataset the error doesn't occur. Because of this I've supposed the problem could be related to temporary spaces or someting similar.